Mixers are used in communications circuits for the purpose of generating a modulated carrier for transmission, demodulating a modulated carrier in reception, or converting a signal at some input intermediate frequency (IF) to another output radio frequency (RF) by multiplying two input signals and thereby generating a third. A number of mixer realizations, both passive and active, are known in the art, and double-balanced mixers are known particularly well due to their advantages in the suppression of unwanted spurious signals and the isolation of any one of three ports to the other two, there generally being two inputs and one output. The Gilbert Cell has been the most widely used active mixer circuit for performing the above tasks, usually incorporated within an integrated circuit. It does, however, possess certain limitations in terms of intermodulation (IM) distortion and noise figure (NF) that precludes it""s use in communications systems having demanding performance specifications. The series-shunt feedback mixer delivers a much improved IM performance, but the lossy nature of the feedback topology does not improve the NF performance. The lossless feedback mixer offers an improvement in noise figure, and this performance can be further improved by a simple modification.
Referring to FIG. 1, a schematic diagram of a lossless feedback double-balanced active mixer is shown in functional form. Here, the mixer is comprised of switching transistors 101, 102, 104, and 105, which are turned on (saturation) and off (cutoff) alternately by a differentially applied local oscillator (LO) signal. By this switching action, a pair of currents generated by driver transistors 103 and 106 are divided into four paths, there being two paths for each of two currents. The currents generated by driver transistors 103 and 106 are the result of an input intermediate frequency (IF) signal applied differentially to the input windings of a pair of feedback transformers 107 and 108. The hybrid transformers 111 and 112 combine the four currents from switching transistors 101, 102, 104, and 105, creating a differential pair of feedback currents 119 and 120, as well as an output RF signal 121. The feedback currents 119 and 120 are coupled to the output windings of feedback transformers 107 and 108, respectively, thereby forming a pair of lossless feedback amplifiers which serve to establish the conversion gain and improve the IM performance of the mixer.
Those familiar with the art will readily understand that the improved NF performance of the lossless feedback double-balanced active mixer is a result of the lack of additional noise sources in the embedding topology. This active mixer offers considerable advantages over the more traditional Gilbert Cell active mixer, especially in terms of signal-handling and performance variations over temperature due to the temperature dependency of the emitter resistance re of the driver transistors, and the tradeoffs that are encountered in receiver and transmitter system design. It further provides an improvement in NF over the Gilbert Cell mixer and the series-shunt feedback mixer.
It is the purpose of this invention to further advance the art of feedback mixers by addressing the sources of noise present in the lossless feedback double-balanced active mixer, and to therefore provide an active mixer of substantially improved NF performance, while at the same time retaining the desireable power consumption, IM performance, and overall sense of simplicity and cost effectiveness of the lossless feedback double-balanced active mixer.
A lossless feedback double-balanced active mixer circuit with improved intermodulation (IM) and noise figure (NF) performance is described which includes a pair of lossless feedback balanced active mixer circuits, each of which includes a differential pair of switching transistors which divide a controlled current into two paths at a rate determined by an input local oscillator (LO). A hybrid transformer in each lossless feedback balanced mixer, consisting of a centre-tapped primary winding and a secondary winding, combines the two currents to provide a recombined amplified IF signal and an output radio frequency (RF) signal. The controlled current is provided by an input intermediate frequency (IF) signal and its relation with the lossless feedback mixer input impedance. Each lossless feedback active mixer circuit further includes a feedback transformer, comprised of an input winding and a tapped output winding, which compares the input IF signal with the recombined amplified IF signal from the hybrid transformers and applies the difference as a correction to the input current, thereby completing a lossless feedback amplifier circuit which improves the IM performance of the mixer circuit. Since the feedback transformer is essentially lossless, it introduces no significant sources of noise to the active mixer circuit, and therefore the NF of the of the lossless feedback active mixer circuit remains unimpaired beyond the NF of the transistors themselves. The NF of the lossless feedback double-balanced active mixer is further improved by minimizing the number of active devices. The connection of the secondary windings of the hybrid transformers of the lossless feedback active mixer circuits effectively cancels the output LO and IF signals and provides and output RF signal.